This invention relates to a novel vinylidene fluoride fluoroelastomer having a high molecular weight. The present fluoroelastomer has both low compression set and high elongation properties and shows good processability in spite of a high molecular weight.
There have been proposed hitherto various kinds of vinylidene fluoride fluoroelastomers characterized in the amount of monomeric units. For example, Japanese Patent Publication No. 7394/1958 discloses a copolymer of 40 to 85 wt % of hexafluoropropylene (denoted as HFP hereinafter) units and 60 to 15 wt % of vinylidene fluoride (denoted as VdF hereinafter) units. U.S. Pat. No. 2,968,649 discloses a fluoroelastomeric terpolymer consisting of 97 to 65 wt % VdF and HFP units and 3 to 35 wt % tetrafluoroethylene (denoted as TFE hereinafter) units, the VdF and HFP units being present in a weight ratio within the range of 2.33:1.0 to 0.667:1.0. Japanese Patent Publication No. 18957/1973 teaches a fluoroelastomeric terpolymer containing 90 to 70 wt % VdF and HFP units and 10 to 30 wt % TFE units and having a weight ratio of VdF to HFP in the range of 1.6:1 to 4:1. U.S. Pat. No. 4,123,603 discloses a fluoroelastomeric terpolymer of 57 to 61 wt % VdF units, 27 to 31 wt % HFP units and 10 to 14 wt % TFE.
Furthermore, there have been proposals to improve the processability of fluoroelastomers by employing specific preparation methods. For example, U.S. Pat. No. 3,801,552 and U.K. Pat. No. 1,384,398 disclose a process for preparing a fluoroelastomer by means of suspension polymerization in an aqueous medium containing liquid monomers or monomers dissolved in a liquid halogenated hydrocarbon in the presence of a polymerization initiator. Fluoroelastomers obtained from this process show improved processability and low Mooney viscosity in comparison with a conventional fluoroelastomer having the same inherent viscosity value. However, although some of these fluoroelastomers show a relatively high molecular weight, the mechanical properties and compression set characteristics thereof are not adequately improved.
U.S. Pat. No. 3,845,024 teaches a continuous cascade process employing two reactors for preparing a fluoroelastomer having a bimodal molecular weight distribution by means of emulsion polymerization. According to the disclosure, the final blended fluoroelastomer which has a bimodal molecular weight distribution consists of a high molecular weight copolymer having an inherent viscosity (dl/g) of 1.5 to 3 and a low molecular weight copolymer having an inherent viscosity (dl/g) of 0.1 to 0.8, while the final fluoroelastomer has an inherent viscosity (dl/g) of 0.4 to 1.5. This fluoroelastomer has improved processability and a relatively high molecular weight in comparison with the conventional fluoroelastomer. However, the mechanical properties and compression set characteristics thereof are not improved to a satisfactory level.
As shown in the prior art described above, compression set and elongation properties are generally incompatible with each other because a decrease in compression set results in a decrease in elongation for a fluoroelastomer. Similarly, a high molecular weight is contrary to good processability. It has, therefore, been difficult to simultaneously satisfy these properties.